1. Field of the Invention
The present invention relates to a method of fabricating a compound semiconductor layer on a substrate by forming a thin layer of a compound semiconductor which material is different from that of the substrate.
2. Description of the Related Art
Compound semiconductors are extensively used in a variety of semiconductor elements and optical devices represented by semiconductor lasers, solar cells and high speed transistors such as HEMT. However, because these devices use a substrate made of a compound semiconductor, they are very expensive. Thus, trials are being made to use, as the substrate, Si, Ge, sapphire and the like which can be economically fabricated in the form of a large diameter crystal, and to make a compound semiconductor layer grow epitaxially on such a substrate.
However, formation of a compound semiconductor layer excelling in crystalline nature is difficult since distortions are likely to develop therein due to the difference in crystal structure, particularly in lattice constant, between the substrate and the compound semiconductor. To cope with such lattice unconformity, various countermeasures have been adopted including distortion relaxation via an annealing treatment after the formation of the compound semiconductor layer (See, for example, Japanese Patent Unexamined Publication JP-A-5-32486 claim 1, p. 3 to 4), and interposition of an intermediate (buffer) layer between the substrate and the compound semiconductor layer whereby the intermediate layer has a lattice constant close to those of the two materials (See, for example, Japanese Patent Examined Publication JP-B-7-60791 claim 1, p. 3 to 4).
The foregoing method such as annealing treatment or buffer layer interposition is effective in the case where the difference in lattice constant between the substrate and the compound semiconductor is relatively small. Specifically, when the difference in lattice constant does not exceed roughly 3%, a compound semiconductor layer having a thickness of about several μm and with substantially a satisfactory crystal quality can be obtained. However, when the difference in lattice constant exceeds 3%, the numbers of dislocation and defect in the compound semiconductor layer become so noticeable that the layer cannot be used as an electronic device. Hence, the actual situation of electronic device manufacture is to select the combination of a substrate with a compound semiconductor wherein the difference in lattice constant does not exceed 3%.